Method of fabricating and treating titanium-aluminum alloy parts to improve them forhigh temperature use



United States Patent METHOD OF FABRICATING AND TREATING TITANIUM-ALUMINUM ALLOY PARTS TO IMPROVE THEM FOR HIGH TEMPERA- TURE USE William W. Gullett, College Park, Md., assignor to Chicago Development Corporation, Riverdale, Md, a corporation of Delaware No Drawing. Application October 17, 1957 Serial No. 690,610

1 Claim. (Cl. 148-63) This invention relates to titanium alloys for use in oxidizing atmospheres at high temperatures. The use of titanium alloys for many purposes has been limited by their absorption of oxygen at temperatures above 1200 F. I have found that alloys of titanium with 620% aluminum and more particularly those with -20% aluminum do not absorb oxygen at elevated temperatures except in a surface layer which does not affect the mechanical properties of the balance of the alloy. Unfortunately such alloys as made from conventional materials and by conventional techniques cannot be hot worked and do not have the necessary mechanical properties at temperatures around 1200 F.

In my copending application, Serial No. 668,770, filed June 28, 1957, I have disclosed that titanium-aluminum alloys containing from 6-20% aluminum can be hot worked in the absence of oxygen but that such oxygenfree alloys are not strengthened by increased aluminum. In that application, I have also disclosed that these alloys may be strengthened by adding up to 2% vanadium.

My present invention consists in forming such low oxygen alloys in the absence of oxygen then rendering them stable in oxidizing atmospheres at elevated temperatures by forming a high oxygen surface layer. This layer may be formed under conditions of use or by preforming under more severe conditions.

The relationship of aluminum content of the alloy to oxygen penetration is shown in the following table. All sample having been heated in air at 1400 F. for 24 hours.

It will be seen that a definite high oxygen layer is formed only in the 6, 10 and 20% aluminum alloys. In alloys lower in aluminum, the penetration of oxygen is not limited to a surface layer. If these oxidized alloys are now heated at 1200 F. in air for 2 weeks, additional oxygen will be absorbed by the 2 and 4% alloys but not by the 6, 10 and 20% alloys.

The oxygen absorption is not affected by adding up to 2% vanadium to the 6, 10 and 20% aluminum alloys, the strength of such alloys is, however, increased in proportion to the vanadium content as disclosed in my copending application above referred to.

Having now described my invention in its more general aspects, I will illustrate it by examples.

Example I In this example I make an alloy of titanium with 10% Al, 1% V and .010 oxygen and substantially no other elements in order to prevent oxygen absorption in depth during exposure of said alloy to oxidizing atmospheres. I forge it into a rod /2 diameter after coating with titanium carbide, by first coating the ingot with graphite then making a cathode in a molten electrolyte composed of 65% SrCl 35% NaCl in which is dissolved 8% Ti as chloride, average valence 2.3 and at least one metal selected from the group of alkali and alkaline earth metals 1% as determined by gas evolution in ferric chloride solution. The rod is then annealed at 800 C.

The rod is turned in a lathe to remove .010 inch; one half is then heated for 2 weeks in air at 1200 F.; at the end of this time an additional .010 inch is turned oil the heated portion of the rod.

The mechanical properties of the two halves are then determined with the following results:

UTS YP Elong.,

percent Unoxidized Rod 175,000 162,000 20.0 Oxidized Rod 174,000 163,000 21.0

It will be clear that the mechanical properties of the alloy have not been impaired by heating at 1200 F. in air for 2 weeks.

Example II I take an alloy of 20% aluminum, 2% vanadium and .010% oxygen balance titanium and proceed as in Example I until the rod is turned and halved. I then take one half and heat in air to 1600" F. for 2 hours, and then in air at 1400" F. for 2 weeks. I then turn ofi. .010" and compare the properties of the two halves. The results follow:

UTS YP Elong., percent Unoxidized Rod 275.000 250,000 10.0 Oxidized Rod 270,000 251,000 0.5

References Cited in the file of this patent UNITED STATES PATENTS 2,754,204 Iafiee et a1. July 10, 1956 FOREIGN PATENTS 525,343 Canada May 22, 1956 OTHER REFERENCES Iron Age, July 27, 1950, pages 60-62. 

